How it will be right to use this combination? I mean some phrase like Se mia foresto ne ŝanĝas ion en onia vivo, tiam mia ĉeesto en sia vivo ne gravas. How can I note that (sia) vivo in the second case is the same that onia vivo in the first case?
The sia in the second part of the sentence doesn’t work, because it always refers back to the subject of the part of the sentence containing the verb, and in this case the subject is mia ĉeesto.
According to PMEG, oni has two meanings, either it can refer to any person in general, or a specific set of hypothetical people. It also says that the general case is more common. I guess therefore one solution could be just to repeat onia in the second part of the sentence. However, in that case it’s not totally clear whether the second onia refers to the general case or the same set of people as in the first case. It could mean something along the lines of “if my presence isn’t important to some people, then it isn’t important to anybody”.
I think you could work around this ambiguity by using ies and ties instead:
Se mia foresto ne ŝanĝas ion en ies vivo, tiam mia ĉeesto en ties vivo ne gravas.